1. Heat the oven to 200C/180C fan/ gas 6Tip the spinach into a colander and pour over boiling water to wilt it, then run under cold water and squeeze out as much water as you can and set aside.2. Quarter the celeriac and cut each piece into thin slices. Butter a gratin dish and tip in the celeriac, spinach and the cream. Season, add the nutmeg and toss everything together. Cover the dish with foil and bake for 30 mins. Remove the foil and toss the celeriac around then cover it again put back in the oven for 30 mins3. Remove the foil again, lay the fish on top and place a small knob of butter on each fillet. Cover with foil again and pop it back in the oven for a final 20 mins until the fish is cooked and the celeriac is tender. Scatter everything with chives and serve.